This invention relates generally to detonators for explosive objects in a subterranean well and an associated method. The invention relates more particularly, but not by way of limitation, to a detonating bar and method for actuating a tubing conveyed perforator in response to hydrostatic pressure in the tubing.
There are various known techniques for detonating a perforator located in a well bore to perforate a formation to be treated or produced. The general type of technique particularly pertinent to the present invention is the type moved into the tubing on which the perforator is conveyed into the well and impacted on a stinger extending above the gun of the perforator. An example of such type is disclosed in U.S. Pat. No. 4,512,418, assigned to the assignee of the present invention.
The particular detonator disclosed in U.S. Pat. No. 4,512,418 has two potential shortcomings. One is that it requires a relatively substantial force to be generated upon impact with the stinger before it can detonate the stinger and the gun. This is because operation of the detonator occurs solely from the impact force, which force must be large enough to overcome any and all retaining forces needed to be broken for operating the detonator. The need for this relatively large force generally necessitates that the prior detonator described in the aforementioned patent be dropped into the tubing, rather than lowered into the tubing on a slick line or the like, because such force cannot be readily created when the detonator is carried into the tubing on the slick line or other mechanical means.
This preclusion of use of a slick line or the like is the second potential shortcoming of the specific prior detonator because for such a detonator, which carries a primary explosive, to be totally safe, it should be carried in the well on some type of carrier structure. This is so that if the detonator does not fire, retrieval from the tubing or well bore can be assured. If the detonator is simply dropped into the tubing or the well bore, it must be fished to be retrieved and fishing is not always successful. When fishing is unsuccessful, a potentially serious hazard is created in that when the tubing is to be pulled from the well bore, it must be done so with the unfired explosives in the perforator intact below the unfired detonator. Should the detonator then become dislodged or otherwise caused to impact on the perforator, such as from jarring forces created when the tubing string is being extracted from the well, the perforator could fire at a location which might damage the subterranean formation or, more significantly, it could fire near or at the surface, thereby posing a hazard to the personnel working at the surface.
Therefore, there is the need for an improved detonator for, and an improved method of, actuating a downhole explosive to overcome the foregoing potential shortcomings. Such an improved detonator and method should not be dependent upon a large impact force to operate. The apparatus and method should also utilize a slick line or similar structure so that the detonator can be readily extracted from the tubing if necessary.